1. Field of the Invention
The present invention relates to an image adjusting circuit of a display monitor and, more particularly, to such an image adjusting circuit which is to compensate for pin/barrel-shaped and trapezoid image distortions by integrating the PWM signal generated from a microcomputer of the display monitor and applying the output waveform to a horizontal size adjusting circuit.
2. Discussion of Related Art
Conventionally, in a display monitor, an electron beam generated from a cathode and passing through a shadow mask strikes phosphors in response to the picture signals supplied from a computer, thereby emitting light to form a projected image on the screen of the display monitor. An example of such a display monitor is shown in FIG. 1.
In FIG. 1, a personal computer 100 is comprised of a CPU 110 for processing a keyboard signal and thereby generating output data, and a video card 120 for processing the data received from the CPU 110 into an RGB video signal and further generating horizontal and vertical sync signals which are to synchronize the RGB video signal.
Display monitor 200 receives the RGB video signal and the horizontal and vertical sync signals from the video card 120 in the computer 100. The display monitor 200 is comprised of a microcomputer 210 receptive to the horizontal and vertical sync signals, and discriminating a resolution; a control button section 220 for generating a screen control signal; a horizontal and vertical output circuit section 230 receptive to the screen control signal and a reference oscillating signal generated from the microcomputer 210, and synchronizing a raster; a video circuit section 240 for processing the RGB video signal received from the video card 120 through amplification and displaying them; and a power supplying circuit section 250 for supplying a driving power to the microcomputer 210, the horizontal and vertical output circuit section 230, and the video circuit section 240.
Following is a detailed description of the respective blocks in the display monitor 200 constructed as above.
Microcomputer 210 which stores all sorts of screen control data is receptive to the horizontal and vertical sync signals from the video card 120, and generates an image adjusting signal and a reference oscillating signal in response to the screen control signal applied from the control button section 220.
Receiving the image adjusting signal and the reference oscillating signal from the microcomputer 210, a horizontal and vertical oscillating signal processor 231 supplies a vertical pulse to a vertical drive circuit 232. The vertical pulse is to control the switching rate of a sawtooth wave generating circuit in response to the horizontal and vertical sync signals received from the video card 120.
As regards vertical drive circuit 232 receptive to the vertical pulse, most widely used are two types of them; one-stage vertical amplification type and emitter follower type. The emitter follower type vertical drive circuit has the base of the transistor therein used as an input with the emitter functioning as an output. Hence, the vertical drive circuit 232 normally performs an operation for the improvement of linear characteristic not of the gain.
The vertical drive circuit 232, after amplification, supplies a current signal to a vertical output circuit 233, which will generate a sawtooth current corresponding to the vertical synchronizing pulse flowing through a vertical deflection yoke (V-DY), determining a vertical scanning period in response to the sawtooth current. In addition, a horizontal drive circuit 234 receives a horizontal oscillating signal from the horizontal and vertical oscillating processor 231, and accordingly, provides a current sufficient to switch the horizontal output circuit 235. Horizontal drive circuit 234 is divided into two classes; in-phase type whose output is ON with the drive terminal ON, and out-of-phase type having the output if OFF with the drive terminal ON.
Upon receipt of the current from the horizontal drive circuit 234, the horizontal output circuit 235 will generate a sawtooth current to the horizontal deflection yoke (H-DY), determining a horizontal scanning period depending on the sawtooth current.
In order to supply a stable DC voltage to the anode of a cathode ray tube (CRT) 244, a high voltage is generated even with a weak collector voltage by use of the feedback collector via a fly-back transformed (FBT) 237 and the harmonic wave resulting from the leakage inductance and the distribution capacity of high-voltage circuit 236.
Thus generated high voltage is applied to the anode terminal 244a of the CRT 244, forming a high voltage across the anodic surface of the CRT 244 so as to adjust the luminance of the RGB picture signals which have been amplified in the video circuit section 240. Simultaneously, the video circuit section 240 has an OSD section 241 receiving an on-screen display (OSD) data generated during the screen control of the microcomputer 210 to generate an OSD gain signal.
This OSD gain signal from the OSD section 241 is sent to a video pre-amplifier 242 together with the RGB video signal from the video card 120. The video pre-amplifier 242 amplifies the RGB video signal to a limited voltage level via a low-voltage amplifier.
For example, a signal less than 1 V.sub.PP is subject to an amplification to the voltage of 4-6 V.sub.PP via the video pre-amplifier 242. This picture signal is further amplified to 40-60 V.sub.PP through a video main amplifier 243 and sent to the cathode of the CRT 244 for displaying an image.
The OSD signal is also amplified via the video pre-amplifier 242 and the video main amplifier 243 to display an OSD data on the screen of the CRT 244. This OSD data displayed on the screen provides the user with information relating to the display monitor 200.
Power supplying circuit section 250, which is to provide a driving voltage required for displaying the RGB picture signals on the screen of the display monitor, receives AC voltage through an AC input 251. The AC level is applied to a degaussing coil 252, which resumes the color blotted due to the influence of the earth magnetic field or external environment.
For this, degaussing coil 252 disperses the magnetic field formed across the shadow mask in CRT 244 in order to prevent the electron beams from being deflected unstably, while the AC voltage is applied to the degaussing coil 252 momentarily for 2-8 seconds.
The AC is normally rectified into a DC through a rectifier 253 and sent to a switching transformer 254. The switching transformer 254 supplies all sorts of driving voltage required in the monitor 200 through a voltage regulator 255. At this stage, where there is no vertical sync signal applied from the video card 120, the microcomputer 210 will send a suspend mode signal to a voltage regulator 255 to interrupt the deflecting voltage.
Pulse-width-modulation (PWM) section 256 controls the switching operation of the switching transformer 254, varying the conduction time through PWM so as to stabilize the output voltage of the transformer.
The microcomputer 210 sets up a power-off mode and a suspend mode depending on the presence of horizontal and vertical sync signals in order to save the power consumed in the display monitor 200.
Normally, such a conventional display monitor 200 as described above compensates an image distortion caused by the structure of the CRT 244. In displaying the RGB picture signals on the screen, CRT 244 has the cathode (not shown) generate thermoelectrons and deflects the electron beams in response to the horizontal and vertical sync signals H/V-SYNC.
Since the cathode is disposed in the center of the quadrilateral screen, however, thermoelectrons deflected towards the outer edge of the CRT 244 are projected to the inappropriate positions, which leads to an image distortion.
An embodiment of a conventional image adjusting circuit to compensate such an image distortion will be described below with reference to the attached drawing.
FIG. 2 is a circuit diagram illustrating an embodiment of the image adjusting circuit of the display monitor shown in FIG. 1. As shown in the figure, microcomputer (210 in FIG. 1) stores an image adjusting data in a built-in or external EEPROM (not shown). The image adjusting data stored in the EEPROM is to compensate image distortion generated in the trapezoid, pin-cushion or barrel form on the screen of the display monitor 200.
The image adjusting data is converted to an analog signal via a digital-to-analog converter, (not shown) and sent to integrating circuits. Out of the image adjusting data, a pin/barrel signal is applied to a first integrating circuit 1 consisting of a first operational amplifier OP1, a capacitor C2 and a first switch SW1.
The pin/barrel signal is matched via capacitor C1 and resistance R1 and applied to the negative (-) terminal of the first OP amplifier OP1 through the first switch SW1 and capacitor C2. The positive (+) terminal of the first OP amplifier OP1 receives a reference voltage divided from a DC voltage V.sub.CC via resistances R2 and R3.
Upon receipt of the pin/barrel signal and the reference voltage, the first integrating circuit 1 has the first switch SW1 open during a period of feedback vertical deflection when a vertical retrace pulse V.sub.ret is LOW. The pin/barrel signal is applied to the negative (-) terminal of the first OP amplifier OP1 via the capacitor C2.
The first OP amplifier OP1 generates the pin/barrel signal as a pin/barrel compensated waveform in response to the reference voltage divided via the resistances R2 and R3. The pin/barrel compensated waveform is subject to a decoupling through capacitor C3 and resistance R4 and applied to the negative (-) terminal of a second OP amplifier OP2.
Further, the positive (+) terminal of the second OP amplifier OP2 receives a reference voltage via resistances R2 and R3.
Upon receipt of the pin/barrel compensated waveform and the reference voltage from the first integrating circuit 1, the second integrating circuit 2 has a second switch SW2 open during a period when the vertical retrace pulse V.sub.ret is LOW. As the second switch SW2 is open, a trapezoid signal is generated from the microcomputer 210, converted to an analog signal through the digital-to-analog converter and supplied to the second integrating circuit 2.
In the second integrating circuit 2 which has received the trapezoid signal, the second switch SW2 is open while the vertical retrace pulse V.sub.ret is LOW. As the second switch SW2 is open, the trapezoid signal is matched through capacitor C4 and resistance R5 and supplied to the negative (-) terminal of the second OP amplifier OP2 according to the charge/discharge of capacitor C5.
Normally, the pin/barrel compensated waveform applied from the first integrated circuit 1 and the trapezoid signal supplied via capacitor C6 are overlapped with each other and applied to the second OP amplifier OP2. The second OP amplifier OP2 which has received the overlapped waveforms and the reference voltage integrates the waveforms according to the charge/discharge of capacitor C5, thereby generating an image adjusting compensated wave through resistance R6 and capacitor C6.
The image adjusting compensated wave is applied to a horizontal size regulating circuit (not shown) and the horizontal output circuit (235 in FIG. 1) for the control of the horizontal deflection. Finally, the pin/barrel and trapezoid distortions in the display monitor 200 are compensated through such a control of the horizontal deflection.
The output waveforms of the conventional image adjusting circuit will be described as follows with reference to FIGS. 3-5.
FIGS. 3A-3E illustrate the waveforms generated from each output terminal of the image adjusting circuit shown in FIG. 2, and FIGS. 4 and 5 illustrate the pin/barrel-shaped and trapezoid distortions in the display monitor. The vertical retrace pulse (FIG. 3A) occurs during a vertical deflection. The barrel (FIG. 3B) and pin (FIG. 3C) pulses are applied to a horizontal size regulating circuit (not shown) so as to compensate the pin-cushion and barrel image distortion as shown in FIG. 4.
As seen from the display monitor in FIG. 4, the dotted lines 2 and 2' indicate the pin-cushion image distortion, i.e., a concavo-concave image distorted from a normal one as indicated by the solid lines 3 and 3'. To compensate for the pin-shaped distortion and drive a horizontal deflection of the distortion-compensated image, the waveform (C) generated from the first integrating circuit 1 is processed through the horizontal size regulating circuit, the horizontal output circuit 236 and the horizontal deflection yoke.
The waveform (B) is applied to the horizontal output circuit (235 in FIG. 1) and the horizontal deflection yoke (H-DY in FIG. 1) in order to compensate for the barrel-shaped distortion as indicated by the convexo-convex dotted lines 1 and 1' in displaying the normal image as represented by the solid lines 3 and 3'.
The trapezoid distortion, as shown in FIG. 5, is indicated by dotted lines 4 and 4', and 5 and 5' that are inclined inward and outward from the solid lines 3 and 3' representing the normal image. To compensate for the trapezoid distortion and drive a compensated vertical deflection, the second integrating circuit (2 in FIG. 2) which has received the trapezoid signal from the microcomputer (210 in FIG. 1) outputs the waveform (D).
The waveform (D) is used to compensate for the trapezoid distortion into the normal image. Further, the waveform (E) is to compensate for only the trapezoid distortion and drive the horizontal deflection through compensated through the horizontal output circuit 235 and the horizontal deflection yoke H-DY.
However, the conventional image adjusting circuit for compensating for the pin/barrel-shaped and trapezoid distortion requires a separate digital-to-analog converter for converting the image adjusting data stored in the microcomputer to an analog DC level, increasing the complexity of the circuit construction, so that it has a limitation in generating the output waveforms through an integration of only the d-c level, with a consequence of deterioration of the efficiency in compensating for the pin/barrel-shaped and trapezoid image distortions.